Purpose The aim of this study was to compare the corneal hysteresis (CH) and corneal resistance factor (CRF) after small-incision lenticule extraction (SMILE) and photorefractive keratectomy (PRK).Patients and methods Fifty-six eyes of 28 patients were included in this study. The study eyes had myopia or myopic astigmatism with spherical equivalent less than or equal to −6 D. They were divided into two groups: group I (28 eyes of 14 patients) was corrected using the SMILE technique and group II (28 eyes of 14 patients) was corrected using the PRK technique. CH and CRF were measured in all eyes preoperatively and postoperatively using the ocular response analyzer instrument. Best-corrected visual acuity, spherical equivalent, and central corneal thickness were documented preoperatively and postoperatively in each follow-up visit. All patients were followed up at 3, 6, and 12 months postoperatively.Results The mean patients’ age was 23.8±4.2 years in group I and 24.1±4.7 years in group II (P=0.85). In the SMILE group, CH changed from 10.57±0.38 preoperatively to 8.6±0.26 at 12 months postoperatively (P<0.001), whereas in the PRK group it decreased from 10.7±0.4 to 8.7±0.26 (P<0.001). CRF changed from 10.21±0.1 to 8.4±0.2 and from 10.2±0.11 to 8.5±0.22 after SMILE and PRK, respectively (P<0.001). There were no significant changes in corneal biomechanical factors between SMILE and PRK after 12 months (P=0.1).Conclusion CH and CRF decreased after SMILE and PRK. There were no differences between SMILE and PRK treatments in postoperative CH or CRF values.

The study of corneal biomechanics plays an important role in the detection of many corneal disorders [1]. Examination of these biomechanical properties is an essential factor in refractive surgery to find any corneal abnormality [2].

The corneal biomechanical properties are detected using the ocular response analyzer (ORA), which is a device that measures both corneal hysteresis (CH) and corneal resistance factor (CRF). CH reflects the corneal viscoelastic properties, and CRF measures the whole corneal rigidity [3],[4],[5].

There is a change in corneal biomechanics after different refractive procedures such as small-incision lenticule extraction (SMILE), photorefractive keratectomy (PRK), and laser-assisted in-situ keratomileusis (LASIK) [6],[7]. The biomechanical properties of the cornea are important factors for the maintenance and stability of a good postoperative quality of vision [8],[9].

A newly developed SMILE technique is a surgical procedure in which an intrastromal lenticule is cut using femtosecond laser and extracted manually through a small peripheral corneal incision. In the SMILE technique, there is no need to create a corneal flap as it is replaced with a cap [10],[11]. Therefore, SMILE is considered a safer surgical technique in comparison with other refractive surgical procedures such as LASIK and PRK [12].

SMILE surgery is considered to have a better effect on corneal biomechanics compared with the LASIK procedure, as in SMILE there is no flap, with normal nonaffected corneal stroma on the lenticule [13].

PRK procedure is considered as an alternative technique to LASIK for surgical correction of errors of refraction [14]. PRK uses the excimer laser to ablate the corneal tissue with subsequent effect on the corneal stroma and damage to Bowman’s membrane. Corneal biomechanical properties decrease after PRK procedure [15],[16].

Different studies concluded that all types of corneal surgeries for the correction of refractive errors are associated with changes in corneal biomechanics, with subsequently decreased corneal stability and corneal deformation [17].

The aim of this work was to compare both CH and CRF after SMILE and PRK techniques.

Patients and methods

Fifty-six eyes of 28 patients were included in this prospective, nonrandomized study. They were divided into two groups: group I included 28 eyes (14 patients) that were corrected using the SMILE technique, whereas group II included 28 eyes (14 patients) that were corrected using PRK procedure.

Inclusion criteria

Myopic eyes or those with myopic astigmatism with stable refraction for at least 1 year were included in the present study. Eyes with spherical equivalent (SQ) less than or equal to −6.00 D with astigmatism less than or equal to −2 D were included in this study. All eyes included in this study had a central corneal thickness (CCT) of 500 μm or more. All patients were older than 18 years.

Exclusion criteria

Eyes with progressive myopia and those with SQ more than −6.00 D or with astigmatism more than 2 D were excluded from the study. Eyes with CCT less than 500 μm were excluded from the study. Patients with ocular disease or with previous intraocular or corneal surgery were also excluded from the study.

Corneal biomechanical properties including CH and CRF were measured using the ORA (Reichert Ophthalmic Instruments, USA). Corneal topography and CCT were investigated and measured using a Pentacam (Oculus Optikgeräte GmbH, Germany).

All patients provided informed consent and the study was approved by the Institutional Review Board and was conducted in accordance with the Declaration of Helsinki and the principles of Good Clinical Practice.

Surgical techniques

SMILE procedures were carried out using the VisuMax Femtosecond Laser System (Carl Zeiss Meditec, Germany). Under topical anesthesia (benoxinate 0.4% eye drops), the femtosecond laser was used to create the lenticule to correct the refractive error for all eyes in group I. The thickness of the cap was designed to be 120 μm, with a diameter of 6.5–7.5 mm. The lenticule was removed manually from the side cut, which was performed at 2 mm from the central position.

PRK procedures were performed in group II eyes under topical anesthesia. The corneal epithelium was removed using alcohol. After removal of the epithelium, the central corneal stroma was ablated using the excimer laser. After laser ablation, mitomycin C 0.02% was applied on the ablated stroma. Thereafter, the stromal bed was irrigated with a balanced salt solution.

Postoperative topical steroid and antibiotic eye drops were used for all eyes in both groups.

Postoperative follow-up

All patients were followed up at 3, 6, and 12 months after the surgery. In each visit, complete ophthalmic examination was carried out, including BCVA and SQ measurement. Corneal biomechanical parameters (CH and CRF) and CCT were measured at each visit using the same Reichert ORA and Oculus Pentacam instruments.

Statistical analysis of the collected data was performed using IBM SPSS Statistics 14.0 (IBM SPSS, Chicago, Illinois, USA). A P value of less than 0.05 was considered statistically significant.

Results

The study included 56 eyes of 28 patients who had either myopia or myopic astigmatism. They were divided into two groups: group I was corrected surgically using the SMILE technique and group II was corrected using PRK.

Group I included 14 patients (28 eyes) with a mean age of 23.8±4.2 years (range: 19–35 years), and group II included 14 patients (28 eyes) with a mean age of 24.1±4.7 years (range: 19–36 years); there was no significant difference between the two groups (P=0.85). Preoperative SQ in group I was −3.75±1.27 D (range: −1.75 to −6.00 D) and the mean BCVA was 0.95±0.06 (range: 0.8–1.0), whereas in group II it was −3.3±1.6 D (range: −1.25 to −6.0 D) and 0.93±0.07 (range: 0.8–1.0), respectively. There was no significant difference between the two groups (P=0.28 and 0.45, respectively). The mean preoperative CCT was 534.4±13.5 μm (range: 511–561 μm) and 533.3±11.4 μm (range: 519–559 μm) in groups I and II, respectively, with no significant difference (P=0.76) ([Table 1]).

Table 1 Preoperative and postoperative data of patients included in the study

All patients were followed up postoperatively at 3, 6, and 12 months; data collected during the follow-up period is presented in [Table 1].

The CH and CRF were measured in both groups preoperatively and postoperatively. In group I, the mean preoperative CH was 10.57±0.38 (range: 10–11.2) and CRF was 10.21±0.1 (range: 10–10.5), whereas in group II it was 10.7±0.4 (range: 10–11.2) and 10.2±0.11 (range: 10.1–10.5), respectively, with no significant difference (P=0.43 and 0.25, respectively).

Both CH and CRF were measured in both groups postoperatively at 3, 6, and 12 months of follow-up. After 3 months, CH and CRF were 8.3±0.25 (range: 8.0–9.0) and 8.17±0.19 (range: 7.9–8.5) in group I and 8.4±0.27 (range: 7.9–9.1) and 8.2±0.23 (range: 7.8–8.6) in group II, respectively, with no significant difference (P=0.28 and 0.45, respectively). At 6 months, CH and CRF were 8.55±0.26 (range: 8.2–9.1) and 8.28±0.18 (range: 7.9–8.6) in group I and 8.6±0.28 (range: 8.1–9.2) and 8.3±0.21 (range: 7.8–8.6) in group II, respectively, with no significant difference (P=0.56 and 0.32, respectively). After 12 months, CH and CRF were 8.6±0.26 (range: 8.2–9.1) and 8.4±0.2 (range: 8.0–8.7) in group I and 8.7±0.26 (range: 8.2–9.2) and 8.5±0.22 (range: 7.9–8.9) in group II, respectively, with no significant difference (P=0.35 and 0.1, respectively). These data are presented in [Table 2] and [Figure 1] and [Figure 2].

The corneal biomechanical parameters decreased after both smile and PRK procedures. The postoperative CH and CRF values at 12 months were significantly lower than the preoperative values in both groups (P<0.001). These data are presented in [Table 3] and [Figure 3] and [Figure 4].

The cornea is the main refractive medium of the eye, and hence its biomechanical properties play an important role in good optical and visual functions of the eye. These properties are essential in visual performance and quality of vision after corneal refractive surgery [18].

Different corneal refractive surgeries (LASIK, PRK, femtosecond LASIK, laser-assisted subepithelial keratectomy, and SMILE) result in alterations in corneal biomechanical parameters due to stromal removal or ablation with subsequently decreased CH and CRF [19],[20],[21].

CH and CRF are corneal biomechanical properties, which reflect the corneal viscoelastic properties [22]. Previous studies evaluated CH and CRF after LASIK and PRK [20]. In the present study, the changes in CH and CRF after SMILE and PRK were compared.

This prospective study included 56 eyes with myopia or myopic astigmatism less than or equal to −6 D, which underwent SMILE (28 eyes) and PRK (28 eyes). The data on the changes in the corneal biomechanical parameters were presented after each procedure.

As the biomechanical properties of the cornea (CH and CRF) are associated with age and many studies concluded that the CH is correlated with the CCT [23], the patients in this study were selected to be of the same age group and of similar CCT (there was no difference between the two groups of the study in age or CCT).

The results of this study supported that both CH and CRF were significantly reduced at 12 months after both SMILE and PRK, with the stability of these biomechanical parameters through the whole period of the follow-up. However, there was no significant difference between the SMILE and PRK groups in CH and CRF changes during the follow-up period.

Wang et al. [24] compared CH and CRF before and after SMILE and femtosecond LASIK in different degrees of myopia. They found that there was no significant difference between the two groups in myopia less than −6 D. However, in eyes with myopia more than −6 D, the CH and CRF decreased significantly more in LASIK than in SMILE cases.

Wu et al. [25] compared CH and CRF after both the SMILE technique and femtosecond LASIK procedure. They found that both CH and CRF were significantly reduced after both procedures, whereas the changes in the corneal biomechanical parameters were lesser after SMILE than after LASIK procedure.

Kamiya et al. [7] compared the postoperative CH and CRF after both PRK and LASIK in myopic eyes. They found that both PRK and LASIK have an effect on the corneal biomechanical properties with a significant postoperative decrease in CH and CRF according to the amount of the myopia. They also found that the decrease in CH and CRF was significantly more after LASIK than after PRK. Hence, they concluded that LASIK is a more invasive technique compared with PRK in the correction of myopia.No flap is created in SMILE procedure, and hence most of the anterior stroma of the cornea is left intact, which results in increased stability of the cornea postoperatively [26], whereas PRK technique involves the removal of a superficial layer of the cornea. However, this study documented that CH and CRF were reduced after both SMILE and PRK, with no significant difference between the two procedures.

The decrease in corneal biomechanics after both procedures was attributed to the fact that CH and CRF values are correlated with CCT, and hence after these refractive surgeries the decreased CCT will cause associated decrease in CH and CRF [27]. Both PRK and SMILE techniques affect the Bowman’s membrane, as PRK technique involves the removal of a part of the corneal stroma with destruction to Bowman’s membrane, and the SMILE technique is associated with microdistortions in Bowman’s layer [28]

Kamiya et al. [29] examined the biomechanical properties of the cornea in myopic eyes after SMILE procedure, and they found that the highest changes in the corneal biomechanics occurred at the first week postoperatively, and they became stable after that.

Moreover, Mastropasqua et al. [30] used Scheimpflug noncontact tonometer to measure the changes in corneal biomechanical parameters after SMILE, and they found that the changes that occurred in biomechanics were during the early (1 week) postoperative period.

The patients in this study were followed up at 3, 6, and 12 months after the surgery. CH and CRF were measured after 3 months of the operation to avoid any increase in intraocular pressure due to postoperative steroid medications, which may have an effect on CH and CRF. The results of this study showed stability of both CH and CRF throughout the follow-up period.

In conclusion, CH and CRF are decreased after both SMILE and PRK procedures, with no significant difference between the two groups as regards the changes in corneal biomechanical factors after the surgery.